Polyether alcohols are important feedstocks in polyurethane foam preparation and the production of substances having interface-influencing properties. Polyether alcohols are prepared usually by ring-opening addition of short-chain alkylene oxides such as ethylene oxide, propylene oxide, butylene oxide, dodecene oxide and/or styrene oxide to low molecular weight alcohols such as butanol and/or allyl alcohol. The catalysts used are usually metallic hydroxides or salts, potassium hydroxide having the greatest practical significance.
The reaction with ethylene oxide proceeds according to the following scheme:

XH refers in the widest sense to H-functional groups on an organic radical R, and RXH is generally present as ROH (alcohol). Alcohols shall also refer hereinbelow to other H— functional substances (RXH where X═O, S, NH or NR′ where R′ is an organic, cyclic or acyclic, optionally substituted radical having from 1 to 18 carbon atoms). However, the alcohols where X═O have gained the greatest signficance in practice as starting substances in the alkoxylation. Frequently, the initially charged alcohol is reacted initially with a base such as KOH to give the corresponding alkoxide and, after removal of the water of reaction formed, the alkoxide is reacted with ethylene oxide. In the present case, the catalyst remains in the reaction mixture as a reactive component and optionally likewise functions as a starter molecule.
When the base is added, an equilibrium forms between alcohol/base and alkoxide/corresponding acid:

The corresponding acid may either remain in the system or else be removed distillatively. When the corresponding acid remains in the system, the base is also alkoxylated. At this point, there is an equilibrium, so that both polyethers of the starter alcohol via the stage of the polyether alkoxide and polyethers of the corresponding base or of the polyether which has already formed from the reaction of the base with alkylene oxide are formed.
The products are mixtures of homologs of the particular starter molecule of different chain length. A significant problem in the alkoxylation is that a large amount of the monomers which have not yet reacted is present in the reactor during the reaction and thus constitutes an increased safety risk. Furthermore, in the batchwise operating mode, the reactor has to be completely emptied, cleaned and charged again in the event of a product change before a new product can be prepared.
In the batchwise operating mode, the product remains in the reactor usually for between 1 and 30 hours, preferably between 3 and 10 hours, so that undesired by-products result. In the case of allyl alcohol-started polymerizations, there is a rearrangement of allyl ether formed during the reaction to the propenyl ether. Furthermore, when propylene oxide is used as a monomer, a rearrangement of the propylene oxide to the allyl alcohol occurs, which functions as a new starter alcohol and influences the composition of the end product and the molecular mass distribution.
In the case of long-chain polyethers, long residence times at high reaction temperatures result in the elimination of water at the chain end and thus in the formation of vinyl ether derivatives. The water formed functions as a starter molecule and increases the proportion of diols in the product.
Polyethers are prepared in industry generally by two different process principles.
In the first process principle, the alkoxylation is carried out in a stirred reaction which is heatable and coolable. The alcohol to be alkoxylated is initially charged together with the catalyst in the stirred vessel and heated. Subsequently, alkylene oxide is metered into the reactor and the vessel contents are mixed with a stirrer. The alkylene oxide is metered under pressure and temperature control. Since the reaction is very rapid and exothermic, intensive cooling is required. This is effected in the case of stirred reactors by an external cooling jacket and/or by internal cooling coils. After the reaction, the reactor is decompressed and unconverted alkylene oxide is removed by applying reduced pressure. The catalyst is neutralized with acid and the resulting salts are filtered off. If appropriate, the resulting salts also remain in the polyether.
In the second process principle, as laid out in EP-A-0 419 419, the alkoxylation is carried out in a loop reactor. To this end, the alcohol to be alkoxylated is initially charged together with the catalyst in a vessel and circulated by pumping through a pump and pipeline. The heat is removed with the aid of an external heat exchanger which is installed in the pump circulation pipeline.
The temperatures during the reaction in both process principles are in the range from about 50 to about 220° C., preferably from about 110 to about 180° C. The pressure is preferably from about 2 to about 40 bar, preferably from about 3 to about 6 bar. The degree of alkoxylation is established by the ratio of alcohol to alkylene oxide and is limited essentially by the construction of the reaction. The duration of the reaction depends upon the reactor size, the effectiveness of the cooling equipment, how well the reactants are mixed and the nature of the desired product. In general, the duration is several hours.
In practice, alkoxylations are carried out batchwise, which leads to variations in quality.
A significant problem is the spontaneous decomposition of ethylene oxide in the gas phase. To prevent this, the ethylene oxide concentration in the gas space of the reactor is reduced with inert gases, for example nitrogen. This forms additional amounts of off gas on decompression of the reactor, and the achievable degree of alkoxylation or the batch size are adversely affected.
As a consequence of the large holdup in the batchwise preparation, relatively large amounts of unreacted alkylene oxides can be collected, which can lead to the reaction becoming uncontrollable.
The batchwise stirred and loop reactors of the prior art a re restricted by a minimum and maximum fill level which in turn limits the achievable degree of alkoxylation. High degrees of alkoxylation therefore entail a plurality of batch reactors connected in series or the use of product precursors, which is very costly and inconvenient.
The literature therefore already describes continuous processes for alkoxylation which are intended to avoid the abovementioned disadvantages.
DE-A-41 28 827 describes a process for the catalyzed alkoxylation of fat derivatives in a falling-film reactor, in which the alkylene oxide in gaseous form is contacted with the liquid in cocurrent. Advantages are a low content of by-products and high safety owing to the very small amount of alkylene oxide in the liquid phase.
DE-A-100 36 602 relates to a microreactor for reactions of gases with liquids. Several plates provided with grooves form capillaries in which the alkylene oxide is contacted with the liquid. The advantage of this reactor over DE-A-41 28 827 is the low falling-film thickness with low mass transfer resistances.
However, it has to be taken into account in this context that the alkylene oxide is in gaseous form in the reactor. Since the alkoxylation is a liquid phase reaction, there first has to be mass transfer from the gas into the liquid phase. This additional mass transport resistance undesirably lengthens the reaction time.
In addition, the presence of an alkylene oxide gas phase is problematic for safety reasons owing to possible uncontrolled decomposition.
DE-A-100 54 462 describes the continuous reaction of fatty alkoxides with alkylene oxides in plate and/or tube bundle heat exchangers under pressure. This type of reaction control is intended to substantially avoid the safety-critical gas phase of alkylene oxides and to ensure uniform product quality in continuous operation.
However, a fundamental problem of the apparatus described becomes evident in the multiple feeding of the alkylene oxides. The multiple feeds result in smaller maximum alkylene oxide concentrations being achieved in the reactor, with the aim of reducing localized overheating at the feeds, reducing the pressure and preventing localized gas phase formation.
However, the need for multiple feeds arises from the limitations of the plate and/or tube bundle heat exchangers used. Even in the plate heat exchangers which are known to be efficient heat transferers, the heat removal is not sufficient to prevent temperature peaks (“hotspots”) in the case of a single feed. This is all the more true of tube bundle heat exchangers. In addition, pressure increases in both apparatus versions lead to increasingly complicated design.
The kinetic potential which arises in theory by the use of high alkylene oxide concentrations can obviously not be exhausted under these conditions.
In addition, DE-A-100 54 462 does not describe the alkylene oxide feeds in detail. It is evident from the figures and the description that there are comparatively extensive, especially poorly cooled (low specific surface area, no adjacent cooling) geometries at these feeds. Without a countermeasure, this leads to an intensification of the hotspots which would be expected in any case in the exothermic alkoxylations.
In the event of faults, for example in the event of an interruption in the product conveying, there is even the risk that the lack of cooling in the region of the mixing zones, owing to the temperature increase in this region which is then virtually adiabatic, leads to a safety problem.
Furthermore, backmixing in the plate heat exchangers causes greatly differing residence times of the individual flow threads, which leads to products having a broad molecular mass distribution.